Agrobacterium tumefaciens causes the disease crown gall in most species of dicotyledonous plants and on some monocots and gymnosperms. Virulent strains of the bacterium contain one of the large class plasmids termed Ti-(tumor inducing) plasmids. During the infection process a portion of the Ti-plasmid, the T-(transferred) DNA, is transferred from the bacterium to the plant where it is covalently incorporated into the nuclear DNA and is expressed in the living plant cell. Among the products encoded by the T-DNA in plant cells are enzymes involved in the production of the phytohormones auxin and cytokinin, and enzymes involved in the production of unusual low molecular weight compounds called opines. The production of large quantities of phytohormones, directed by the T-DNA, results in the uncontrolled proliferation of plant tissue leading to the growth of crown gall tumors. Opines made by and secreted from these tumors are utilized by the inciting bacteria as a carbon and sometimes a nitrogen source. Thus the transfer to and the expression of the T-DNA in plant cells allows Agrobacterium species to genetically engineer the host plant to make compounds (the opines) that, with a few exceptions, only it can utilize. The bacterium thereby creates a niche for itself in the rhizosphere.
The Agrobacterium system represents a natural situation by which genes are transferred from a bacterium to a plant cell. This system has therefore been modified in many laboratories to deliver novel gene combinations to plant cells, and as such, is currently the most efficient way to transfer foreign genes into the genome of certain plant species. Thus the T-DNA region of the Ti-plasmid of an Agrobacterium species is modified utilizing art-recognized recombinant DNA techniques to delete tumor inducing genes and to include inserted foreign genes coding for proteins and protein systems which, when expressed, imparts favorable phenotypic characteristics to the modified plant species.
The molecular events leading to the excision of the T-DNA from the Ti-plasmid and the transfer and integration of the T-DNA into plant cells is still poorly understood. It is known, however, that the process is initiated when the bacterium species receives certain "chemical signals" from the wounded plant cell. Among the chemicals exuded by wounded plant tissue are phenolic compounds, including acetosyringone. The bacterium responds to such chemicals by activating certain genes called vir genes, on the Ti-plasmid. Among the compounds encoded and expressed by the induced vir genes is an endonuclease that is involved in the excision of T-DNA from the Ti-plasmid. Upon induction by one or more signal compounds from wounded plant cells, single-stranded DNA derived from the T-region of the Ti-plasmid can be found in the Agrobacterium. These molecules, termed T-strands, have been proposed to be the intermediate molecules that are transferred to the plant cells from the bacterium. This model is based upon the mechanism by which bacteria mate, or conjugate, plasmids with other bacteria. The transfer of the T-DNA from Agrobacterium to plant cells has therefore been likened to the transfer of DNA between bacterial cells.
The Agrobacterium system plays a crucial role in present day research and development efforts in the area of plant genetics. Yet, the Agrobacterium system has its inherent limitations. Transformation efficiencies are not always what plant scientists would like them to be. Moreover, there are may so-called recalcitrant plant species which are difficult to transform utilizing the Agrobacterium system. A method for enhancing plant cell transformation efficiency would have many positive implications for plant genetic engineering.